The present invention relates to a rare earth-based permanent magnet of high corrosion resistance or, more particularly, to a rare earth-based permanent magnet mainly consisting of a rare earth element, iron and boron and imparted with high corrosion resistance by providing a highly corrosion-resistant coating layer on the surface thereof as well as to a method for the preparation of such a rare earth-based permanent magnet of high corrosion resistance.
By virtue of the excellent magnetic properties and high economical merits for the high performance, the application fields of rare earth-based permanent magnets are rapidly expanding year by year mainly in the field of electric and electronic instruments so that an important issue in this field is to further upgrade the rare earth-based permanent magnets.
Among various types of rare earth-based permanent magnets currently under practical applications, the permanent magnets formed from a ternary alloy of a rare earth element, iron and boron, referred to as a R--Fe--B alloy or magnet hereinafter, in which R is a rare earth element including yttrium and the elements having an atomic number of 57 to 71, constitute the major current because, besides the very superior magnetic properties, the rare earth element R in the R--Fe--B alloy can be neodymium which is, as compared with the earlier developed rare earth-cobalt magnet, in which the rare earth element is mainly samarium, by far more abundant as the natural resources than samarium and hence less expensive and the relatively expensive metal of cobalt need not be employed as an alloying element. Accordingly, the application fields of the R--Fe--B permanent magnets are expanding not only as a substitute for the rare earth-cobalt magnets used heretofore in compact-size instruments constructed by using very small permanent magnets but also in the field where the magnet constructing the magnetic circuit was a large-size inexpensive permanent magnet of low magnetic performance, such as hard ferrite magnets, or an electromagnet.
As a counterbalancing disadvantage to the above mentioned great advantages, the R--Fe--B magnets in general have a serious problem of low corrosion resistance, due to the reactivity of the rare earth element and iron as the principal ingredients, readily to be oxidized in the air, in particular, containing moisture resulting in a decrease in the magnetic performance of the magnet and possible contamination of the ambience by the oxidized matter eventually falling off the magnets.
Therefore, various proposals and attempts were made heretofore for the improvement of the corrosion resistance of the R--Fe--B magnets by the surface treatment including coating of the surface with a resin-containing coating composition, dry-process metallic plating by the method of, for example, ion plating, wet-process metallic plating to form a plating layer of nickel and so on. These surface treatment methods in the prior art are in general very complicated and time-consuming unavoidably leading to a remarkable increase in the overall manufacturing costs of the R--Fe--B magnets.